1. Field of the Invention
The present invention relates in general to a system or line adapted to prepare a casting mold using a self-hardening sand mixture containing an organic binder, and more particularly to an improved mold preparation system capable of suitably controlling the rate of hardening of the self-hardening sand mixture.
2. Discussion of the Prior Art
A self-hardening casting mold using an organic binder for hardening the molding sand has been widely used for casting medium- and large-sized articles weighing 50Kg or more, such as cast iron parts of machine tools. The use of such an organic binder permits the patterned sand mass to be hardened into a desired mold, without applying heat thereto, and this mold preparation process is sometimes referred to as "no-bake" method. Usually, a furan resin (thermosetting polymer made from furfuryl alcohol) is used as the organic binder mixed in the sand. An acidic hardening catalyst (hardening agent) is generally added to a liquid condensation product resin having a relatively small molecular weight, to initiate a condensation reaction for progressively increasing the activity of the resin, and eventually maximizing the binding force by three-dimensional cross linking, so that the molding sand is firmly packed as a casting mold. In producing a cast article by using such a self-hardening mold, a suitably prepared molten metal is poured into the mold. After the molten metal in the mold is solidified, the mold is opened or broken to remove the casting, and the sand remaining on the casting is removed. The sand of the broken mold is subjected to a reclaiming process in which cast iron fins, flashes or particles or other undesired metallic inclusions are magnetically separated from the sand.
The casting by a self-hardening mold using an organic binder as indicated above has the following advantages: (a) reduced number of steps required for preparing the mold, due to elimination of baking or heating of the sand because of complete hardening at the room temperature of the sand mixture containing an organic binder; (b) relatively high strength of the mold, and improved dimensional accuracy of the cast article obtained from the mold; (c) reduced amount of silica sand required, and reduced environmental pollution due to the waste products in the casting operation; and (d) relatively easy breakage of the sand mold after removal of the cast article, and as high as 90-95% reclaiming of the sand for repetitive mold preparation and casting cycles. For these advantages, the self-hardening casting mold is presently used in many foundries, for casting a wide variety of components for industrial machines and machine tools.
While the self-hardening casting mold has such advantages as described above, the cost of manufacture tends to be high, due to the use of an organic binder resin and a hardening agent, which are comparatively expensive. Various solutions to this problem have been proposed so far. In this connection, it is noted that where the cast products are components such as machine tool parts which are produced in a relatively small lot size, the sand-metal ratio S/M (S and M respectively representing the weights of the sand and metal materials used for preparing the cast product) tends to be relatively high, thereby increasing the cost of the cast products obtained by the prepared mold, since a relatively large volume of sand should be packed in a relatively large metal flask or molding box, in not a few cases. Namely, the use of metal flasks exclusively designed for respective small lots of cast products is not economical, and requires a large storage space. Therefore, the same metal flask should generally be used for the cast products having different sizes. If the flask designed for a large cast products is used for producing a small cast product, the volume of the sand that should be packed in the flask to prepare a casting mold for that cast product is inevitably larger than is actually required.
To lower the sand/metal (S/M) ratio in preparing the casting mold, it is proposed to reduce the amount of the casting sand, by using a mass of ceramic balls or other material which replaces a portion of the sand mass which should fill the appropriate parts of the interior of a metal flask or molding box. Namely, the mass of the ceramic balls or similar material is embedded in the surrounding mass of the sand providing a mold surface which contacts the melt to be poured in the prepared mold and which defines a mold cavity corresponding to the cast product to be produced by the mold.
In the self-hardening casting mold as described above, the strength of the mold structure depends upon the binding force produced by a three-dimensional cross linking phenomenon which occurs as a result of the condensation reaction of an organic binder and a hardening agent which are added to the sand. Since the rate of the condensation reaction is greatly influenced by the temperature of the sand mass, the rate of hardening of the mold sand and the time required to complete the desired casting mold from the sand mass considerably vary with a change in the ambient temperature at the time of preparing the mold.
In fact, it is difficult to hold the ambient temperature within a foundry at a constant level year round, for maintaining the metal flasks and patterns at a constant temperature. There may be a temperature difference of 30.degree. or more between the summer and winter seasons. In a casting operation for producing castings in a relatively small lot size using different flasks and patterns for respective different molds (for different castings to be produced), the temperatures of the flasks and patterns are held relatively constant. However, the temperatures of the flask and pattern tend to gradually increase in a casting operation in a medium lot size, in which the same flask and pattern are repeatedly used for producing different molds for different castings, at relatively short time intervals, with the flasks and patterns being removed from the prepared mold for preparation of the next mold. Thus, the hardening time of the sand mold fluctuates, causing a time schedule in a series of casting operation which includes the preparation of the mold and the pouring of the melt in the prepared mold.
It is therefore necessary to minimize the fluctuation of the required hardening time of the casting mold. On the other hand, the hardening time of the molds should be controlled to meet the requirements in the actual casting operation, such as the required number of cast products per day, and the required delivery of the products. Conventionally, different hardening agents (e.g., mixtures of sulfonic acids and additives) are selectively used and the amount of the selected hardening agent is determined, depending upon the required hardening time.
Suppose a casting mold is sufficiently hardened in about 10-15 minutes in the summer time, but in more than 20 minutes in the winter time, if the pattern, metal flask, the size of the flask, the temperature's of the pattern, flask and sand, the organic binder and the hardening agents are the same in the summer and winter times. In this case, the hardening time in the winter time may be made almost equal to that in the summer time, by increasing the amount of the hardening agent, or using the hardening agent different from that in the summer time if the increase in the amount of the same hardening agent does not attain the desired result.
However, the above conventional method using the different hardening agents is complicated in controlling the hardening time and is difficult to be practiced to achieve the desired result. Further, if an inadequate hardening agent is erroneously used, the entire casing process including the preparation of a casing mold gets out of order. The use of many kinds of hardening agents requires respective storage reservoirs that should be maintained for immediate use. Where a casing mold to be prepared has a large size, it takes as along as 10 minutes or so to pack the sand in the flask, since the surface layer of the sane contacting the pattern should be evenly formed. Moreover, the hardening of the sand may start during the packing of the sand, in the hardening agent is added in a large amount. In this case, the mold preparation efficiency and the quality of the prepared mold are not satisfactory.
If the hardening time of the sand is controlled by regulating the temperatures of the sand, pattern and flask are controlled by application of heat thereto, a relatively large heating device is required, and the sand is heated from the very beginning of the packing in the flask, causing undesired early commencement of hardening of the sand. In this sense, this procedure is not practically available.